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The Exploration of Near-Earth Objects
Committee on Planetary and Lunar Exploration
Space Studies Board
Commission on Physical Sciences, Mathematics, and Applications
National Research Council
NATIONAL ACADEMY PRESS
Washington, D.C.
1998

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NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.
The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce Alberts is president of the National Academy of Sciences.
The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering.
The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.
The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Bruce Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.
Support for this project was provided by Contract NASW 96013 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor.
International Standard Book Number 0-309-06083-4
Cover: An artist’s concept of the Near-Earth Asteroid Rendezvous (NEAR) spacecraft in the vicinity of asteroid 433 EROS. Courtesy of NASA and the Applied Physics Laboratory.
Copies of this report are available free of charge from
Space Studies Board
National Research Council
2101 Constitution Avenue, NW
Washington, DC 20418
Copyright 1998 by the National Academy of Sciences. All rights reserved.
Printed in the United States of America

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Dedicated to the memory of Eugene M. Shoemaker (1928–1997), who was instrumental in the formulation of this report

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Foreword
Comets and asteroids are in some sense the fossils of the solar system. They have avoided most of the drastic physical processing that shaped the planets and thus represent more closely the properties of the primordial solar nebula. What processing has taken place is itself of interest in decoding the history of our solar neighborhood. Near-Earth objects are also of interest because one or more large ones have been blamed for the rare but devastating events that caused mass extinctions of species on our planet, as attested by recent excitement over the impending passage of asteroid 1997 XF11.
The comets and asteroids whose orbits bring them close to Earth are clearly the most accessible to detailed investigation, both from the ground and from spacecraft. When nature kindly delivers the occasional asteroid to the surface of Earth as a meteorite, we can scrutinize it closely in the laboratory; a great deal of information about primordial chemical composition and primitive processes has been gleaned from such objects.
This report reviews the current state of research on near-Earth objects and considers future directions. Attention is paid to the important interplay between ground-based investigations and spaceborne observation or sample collection and return. This is particularly timely since one U.S. spacecraft is already on its way to rendezvous with a near-Earth object, and two others plus a Japanese mission are being readied for launch. In addition to scientific issues, the report considers technologies that would enable further advances in capability and points out the possibilities for including near-Earth objects in any future expansion of human exploration beyond low Earth orbit.
CLAUDE R. CANIZARES, CHAIR
SPACE STUDIES BOARD

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Preface
Asteroids and comets continually pass by Earth, sometimes at uncomfortably close distances. Impacting objects in the geologic past have created large craters and may have caused the extinction of many living organisms. Over the past decade, scientific and popular interest has grown in assessing the likelihood that Earth may be struck in the future by large meteoroids, commonly known as near-Earth objects (NEOs). In 1990, Congress asked the National Aeronautics and Space Administration (NASA) to study the danger. NASA responded with two reports in 1992, calling for increased efforts to locate NEOs and to address issues of hazard mitigation.1,2 A second request from Congress to NASA in 1994 sought a plan for discovering all NEOs larger than 1 km in diameter within a time period of 10 years, as a cooperative effort among NASA, the U.S. Air Force, and international partners. A responding report was released in 1995.3 Several telescope facilities and new instruments now coming into operation will dramatically increase the rate of discovery of NEOs and determine their orbits. This program of intensified discovery efforts offers a unique opportunity to broaden scientific understanding of the distribution, composition, and origin of the population of small bodies in interplanetary space.
Previous reports of the National Research Council have stated that asteroids and comets offer important constraints on the early history of our planetary system, and comets, in particular, have highest priority for scientific study.4 Moreover, the Space Studies Board and its committees have stressed the appropriateness of initiating a program of asteroid and comet study that includes both reconnaissance and exploration phases.5 The most accessible of these bodies, both for observation by ground-based telescopes and for study by spacecraft, are to be found among the NEOs.
1
D. Morrison, ed., The Safeguard Survey: Report of the NASA International Near-Earth Object Detection Workshop, Jet Propulsion Laboratory, Pasadena, Calif., 1992.
2
J.G.D. Rather, J.H. Rahe, and G. Canavan, Summary Report of the Near-Earth Object Interception Workshop, NASA, Washington, D.C., 1992.
3
Solar System Exploration Division, Office of Space Science, Report of the Near-Earth Objects Survey Working Group, NASA, Washington, D.C., 1995.
4
Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, p. 3.
5
Space Science Board, National Research Council, Strategy for the Exploration of Primitive Solar-System Bodies—Asteroids, Comets, and Meteoroids: 1980–1990, National Academy Press, Washington, D.C., 1980.

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Against this background of renewed interest in the study of near-Earth objects, the Space Studies Board charged the Committee on Planetary and Lunar Exploration (COMPLEX) to review current knowledge of NEOs derived from ground-and space-based studies and to answer the following questions:
What is the present understanding of the origin, composition, and physical characteristics of near-Earth objects?
What is the expected level of understanding of NEOs in the next decade?
What levels of ground-based telescopic observation are needed to increase our understanding of targets of high scientific interest?
What are the likely opportunities for low-cost flyby, rendezvous, landing, and sample return missions to these bodies, and to what degree will these missions address fundamental scientific issues?
This project was formally initiated in May 1996, and the bulk of the material was written in early and mid-1997. This material was extensively revised and updated in the late summer of 1997. Although many COMPLEX members past and present worked on this report, the bulk of the task of assembling their many individual contributions was performed by Harry Y. McSween and Eugene Shoemaker with the assistance of James Arnold, Richard Binzel, and Alan Tokunaga. The work of the writing team was made easier thanks to the invaluable assistance rendered by Alan Harris (Jet Propulsion Laboratory).
This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council's (NRC's) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. COMPLEX thanks reviewers Donald Hunten (University of Arizona), Clark Chapman (Southwestern Research Institute), Margaret Kivelson (University of California, Los Angeles), George Wetherill (Carnegie Institution of Washington), and John Wood (Harvard-Smithsonian Center for Astrophysics) for many constructive comments and suggestions. Responsibility for the final content of this report rests solely with the authoring committee and the NRC.